Is a Confirmation of Negotiations (email or letter) required to be submitted by the contractor to the KO upon completion of negotiations to confirm the agreed upon amount as a result of concluded negotiations? If not, was it ever a regulation?
There's nothing in Title 48 of the Code of Federal Regulations (Federal Acquisition Regulations System) that requires contractors to submit a "confirmation of negotiation". The only instance of this term ever appearing in Title 48 was in the Environmental Protection Agency Acquisition Regulation (EPAAR) at 1515.611, where it was used to mean "best and final offer":
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"The Contracting Officer shall establish a common cut-off date for receipt of revised proposals and/or confirmations of negotiations(best and final offers) upon completion of negotiations."
The term seems obsolete as it no longer appears in the EPAAR. I would suspect that if someone were using the term "confirmation of negotiation", they would referring to what is now called a "proposal revision".